SIMULATE-5K is an advanced, two-group nodal code for transient analysis of both PWRs and BWRs. SIMULATE-5K delivers neutronic and thermal-hydraulic analysis with licensing-grade accuracy over a wide range of dynamic applications.


S5K leverages the power of SIMULATE, the industry standard in nodal reactor analysis. Including cutting-edge neutronic methods and advanced engineering features, S5K delivers unparalleled fidelity with production-level run times.

S3K solves the transient three-dimensional, two-group neutron diffusion equations, including a six-group model for delayed neutron precursors. Intranodal flux and power distributions within each node are used to compute the power, fuel temperatures, and enthalpies for every axial level of every fuel pin in the core during transients.

Each fuel assembly in the core is explicitly represented in the core thermal-hydraulics. For BWR operational transients, vessel and steam line models are capable of simulating acoustic effects in the steam line due to sudden valve closures or openings.

Proven Results

SIMULATE-5K (S5K) is built on the solid foundation of SIMULATE, the industry-leading steady-state nodal reactor analysis code.

By explicitly modeling the detailed assembly-by-assembly neutronic and thermal-hydraulic behavior of the reactor core, S5K can be used to analyze a variety of core transients.

S5K has been successfully used by clients for stability analysis, reactivity insertion analysis, dynamic rod worth calculations, and past operational events. Originally designed as a best-estimate tool, S5K is licensed for several transient applications by the United States Nuclear Regulatory Commission.

PWR & BWR Applications

S5K can provide licensing-grade support for analyzing
PWR and BWR specific safety analyses, such as: 

Ejected rods and inadvertent bank withdrawals (RIA)

Coolant inventory or flow change transients using
the two groups of pumps

Boron dilution accidents

Dropped rods and inadvertent bank
withdrawals (RIA)


Pressurizations created in the steam dome and/or
the steam lines with valve control and actuation

Dropped rods

Coolant temperature changes and instabilities that
occur during a flow decrease/temperature events
without the need to re-enter data

Whole-Plant Transients

S5K has been coupled with system codes like RELAP and TRACE. The powerful neutronics engine inside S5K guarantees a high-fidelity, 3D core neutronic solution in whole-plant transient simulations.


Address Emerging Issues

S5K helps operating utilities address emerging regulatory issues like pin enthalpy and delta-CPR limits. An explicit fuel pin conduction model explicitly tracks the complete radial distribution of fuel temperatures and enthalpies in every fuel pin in the core throughout a transient. S5K is capable of evaluating margin to thermal limits (such as delta CPR) during plant transients, providing licensing-grade support to core designers.

Ease of use

The S5K input format is simple to use, allowing free-format input capable of modelling complex core transients. With practical defaults for PWRs and BWRs, robust error checking, and seamless interfaces to other Studsvik core analysis software, S5K allows engineers to spend their time analyzing, not troubleshooting.


With a robust neutronics engine and advanced thermal hydraulic capabilities, S5K is well-suited for transients with a strong neutronic/thermal-hydraulic coupling. Its seamless linkage with SIMULATE makes S5K the perfect tool for the study of operational transients typically analyzed on a cycle-specific basis. Core designers simply generate the proposed core model, and direct S5K to evaluate the prescribed set of transients as part of the core reload design licensing process.


S5K is available for all standard computing platforms running most modern 32- and 64-bit operating systems. Linux, Windows, and UNIX architectures are all acceptable environments for S5K.


For further information, please contact us.